It is time to go back or rather to really go there.
NASA's next big space mission should focus on the planet Uranus.
This is at least the recommendation made, Tuesday, April 19, by the American National Academies of Sciences in their ten-year report on the space priorities of the United States.
Advice that has, in the past, always been followed by the American space agency.
Man has visited this very distant neighbor of the Earth only once, which is the penultimate planet of the solar system, just a little closer to the Sun than Neptune.
It was the 'Voyager 2' probe which approached it for a few hours, on January 24, 1986. In other words, we know practically nothing about Uranus.
Uranus, a unique planet in more ways than one
It is defined as an ice giant that would be the coldest planet in the solar system with an atmospheric temperature of around -220°C.
It is also known that a year on Uranus – the time it takes for it to orbit the sun – lasts 84 Earth years.
According to the few data collected – whether by the 'Voyager' probe or telescopic observations – its surface is not solid and there are oceans of liquid diamonds.
"In reality, we are not sure of its composition and its name of ice giant may be usurped", tempers Ravit Helled, Israeli planetary scientist in the department of astrophysics at the University of Zürich, contacted by France 24.
These unknowns are one of the main reasons justifying a great mission to travel to Uranus.
While missions to Mars or the Moon have multiplied and we are beginning to collect precise information on other stars as well as on exoplanets, there are still almost absolute gray areas in our own solar system. .
"It's as if we were telling you that there is still an unknown ocean on Earth, wouldn't you want to explore it?" asks Laurent Lamy, astrophysicist at the Observatory of Paris, contacted by France 24.
This natural curiosity for our "immediate" surroundings – a very relative notion since Uranus is located between 2.6 billion and 3.2 billion kilometers from Earth – is also nourished by the unique characteristics of the planet.
In particular its rotation: it turns on itself around a horizontal axis and not vertical, like all the other known planets.
As a result, it looks like it's rolling like a ball as it orbits the sun.
A quirk which "would result from a collision with another celestial body, according to the most commonly accepted theory", explains planetary scientist Ravit Helled.
This cosmic shock could even have moved Uranus from its initial orbit.
“We also don't know if it was formed in situ – that is to say where we see it today – or elsewhere”, specifies the Israeli astrophysicist.
It's not just the planet itself that interests scientists.
These many moons – there are 27 of them all bearing the names of Shakespearean characters and from the works of the British poet Alexander Pope – also conceal many mysteries.
Some appear, for example, to be ocean worlds that could harbor life forms and "exploring them would allow us to learn more about potentially habitable places in our galaxy", says Chloe Beddingfield, an astronomer at the Ames Research Center in NASA (in California), interviewed by the Space.com site.
A "missing link"
Uranus also represents - with Neptune - "a missing link in our understanding of the planets that exist in space", assures Laurent Lamy.
They have often been called "mini-Saturns" or "super-Earths", because they are of an intermediate size (about four times that of Earth).
But in reality, it's not just a question of size, they are the only two representatives in our solar system of a family of planets apart, governed by their own rules.
The importance of these "Uranus-like" planets has only grown with the discovery of exoplanets [which lie outside our solar system].
These observations demonstrated that "planets of a size and density comparable to Uranus seem to be very common in space", notes Ravit Helled.
More, in any case, than Earth-like planets or giants of the caliber of Saturn.
Therefore, an orbital mission around Uranus "would make it possible to complete our understanding of the variety of planetary systems accessible in our solar system and to have a relevant reading grid for analyzing more distant systems", summarizes Laurent Lamy.
If Uranus is so unique and could prove to be the key to better understanding a large number of exoplanets, why did you wait so long to decide to go there?
“It is technologically very complicated to go to a planet in the outer solar system [from Jupiter] and we are only beginning to be able to do it,” notes Ravit Helled.
A long and expensive bet
It's already quite an adventure to go there.
Prepare a mission for the still unknown depths of the solar system – decide on the scientific objectives, the most suitable tools, the launcher for the probe etc.
– should take about ten years, according to the report of the American National Academies of Sciences.
The trip itself should take another ten years at least… Whereas it only took four days in 1969 to get to the Moon.
Finally, the probe would probably remain in orbit for another decade in order to make the trip profitable and obtain the maximum amount of data.
It is therefore necessary to provide a source of energy that lasts as long without the risk of breaking down or being damaged.
"This is an important technological challenge and the best solution seems to be the atomic pile. This is also one of the reasons why NASA, which has this technology called radioisotope thermoelectric generator (or RTG), is launches into the adventure before Europe, which has been thinking for a decade about exploring Uranus, but does not have an atomic battery", notes Laurent Lamy of the Paris Observatory.
It is therefore a very long-term mission that will be expensive.
NASA estimates that such a project should cost at least $4.2 billion… for results that may not materialize for decades.
And it is perhaps, ultimately, one of the most daring bets of NASA and the Academies of Sciences: in a world increasingly dominated by the imperative of immediacy where everyone wants everything right away , launching a mission for Uranus would prove that there are still areas in which we can take our time to advance human knowledge.
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